Over three decades of development, surface plasmon resonance (SPR) technology has become a standard technology for label-free characterization of bimolecular interactions. One of the its advantage is that the SPR sensors can provide real-time quantitative measurement by observing amplitude, angular reflectivity, spectral absorption dip or corresponding phase shift of the SPR influenced light (Ann. Phys. (berlin), 524, 637-662, 2012). Recent research efforts have been focused on the development of a SPR sensor with a wide dynamic range and a high sensitivity.
Several so-called wide dynamic range phase SPR sensors have been developed. U.S. Pat. No. 7,027,676 by Van Wiggeren et al. proposed a method to replace the Zeman's laser on optical heterodyne phase detection scheme by using an optical polarization delay unit and tunable laser. As it scans across a range of frequencies of the light source, a frequency offset between the p- and s-polarized light occurs. A modulated optical detection signal can be produced by interference of the p- and s-polarized light. U.S. Pat. No. 8,169,617 by Ho et al. proposed a new design combining phase detection and angular interrogation to simultaneously achieve high sensitivity and a wide dynamic range.
Later, U.S. Ser. No. 13/113,837 by Wu et al. proposed a white light SPR interferometry scheme to achieve both high sensitivity and wide dynamic range via a different mechanism. In their setup, a Michelson interferometer is employed as a phase detecting unit on both p- and s-polarized light. In essence, the design has two interferometers operating independently on the p-polarization and s-polarization components with identical paths. With the SPR sensor head incorporated to one arm of the interferometer, the researchers have achieve differential SPR phase detection by measuring the phase of the p-polarized and s-polarized light separately using a polarization separating Wollaston prism. Since the SPR effect only affects the p-polarization while leaves the s-polarization unchanged, simple subtraction between the phase values measured from the two polarizations will yield a true SPR phase, which is free from any unwanted signals caused by environmental fluctuations or the system itself.